A description will be given of the configuration of a conventional switching element which utilizes electro-chemical reactions (hereinafter simply called the “switching element”).
FIG. 1 is a schematic cross-sectional view showing an exemplary configuration of a switching element.
As shown in FIG. 1, the switching element comprises first electrode 101, second electrode 102, and ion conductive layer 103. In FIG. 1, ion conductive layer 103 which contains oxygen is sandwiched between first electrode 101 and second electrode 102.
A brief description will be given of the operation of the switching element in the configuration shown in FIG. 1.
As first electrode 101 is grounded and second electrode 102 is applied with a positive voltage, metal ions are supplied from second electrode 102 through an electro-chemical reaction. When the metal ions migrate to first electrode 101 in accordance with an electric field and receives electrons from first electrode 101, a metal is deposited in the ion conductive layer through an electro-chemical reaction. As the deposition continues, first electrode 101 and second electrode 102 are interconnected by the metal, causing the switching element to have a low resistance and thus transition to an ON state. On the other hand, the metal deposited in the ion conductive layer by applying a negative voltage to second electrode 102 transforms into metal ions through a reverse reaction, and the metal ions return to original second electrode 102. In this event, the switching element has a high resistance, and thus transitions to an OFF state. In this way, the switching operation is enabled with electric characteristics between the two electrodes which differ in the ON state and OFF state.
Since the electro-chemical reaction is utilized, second electrode 102 is preferably made of an ionization-prone metal material which can supply metal ions to ion conductive layer 103. Contrary to this, first electrode 101 is preferably made of a metal material which is resistant to ionization.
In this regard, while a two-terminal switching element is shown herein, a three-terminal switching element is disclosed in International Publication WO 2005/008783.
Next, a description will be given of an application of the switching element shown in FIG. 1 to a semiconductor integrated circuit such as a programmable logic, a memory or the like.
FIG. 2 is a schematic cross-sectional view showing an exemplary configuration of a semiconductor integrated circuit which is provided with the switching element. As shown in FIG. 2, the semiconductor integrated circuit comprises transistor layer 122 including transistor devices formed on a main surface of substrate 120, wiring layer 124 for electrically connecting the transistor devices, and passivation film 126 in order. Switching element 110 is arranged in inter-layer dielectric film 132 within wiring layer 124. Also, wiring layer 124 is provided with wiring via 134 for connecting different wires with each other. A gate electrode of transistor layer 122 is connected to wire 130 through a plug, a wiring via and the like.
As shown in FIG. 2, wiring layer 124 provided with switching element 110 is formed with wiring via 134, wire 130, and inter-layer dielectric film 132, and passivation film 126 is formed on inter-layer dielectric film 132. In this way, for incorporating a switching element which utilizes electro-chemical reactions in a semiconductor integrated circuit such as a programmable logic, a memory or the like, a number of processes must be taken into consideration in addition to switching element manufacturing processes.
Semiconductor integrated circuit manufacturing processes are roughly classified into a front end process and a back end process. The former is a process for mainly forming semiconductor devices such as transistors, resistors, capacitors and the like near the surface of a silicon substrate. On the other hand, the latter is a process for forming wires for connecting between transistor devices, passivation films, and the like. In the front end process, thermal treatments are performed at 1000° C. or higher in such processes as formation of oxidized films through thermal oxidization, activation of impurity ions, and the like. On the other hand, in the back end process, thermal treatments are approximately at 350° C. to 400° C. in an inter-layer dielectric film formation, an anneal process and the like.
The switching element which utilizes the electro-chemical reactions cannot endure high-temperature treatments in the front end process because it is provided with metal electrodes as described above. For this reason, it is preferably fabricated in the back end process. Thus, the switching element is formed in the inter-layer dielectric film within wires, as shown in FIG. 2.